A conventional modulated infrared type detecting device has its modulating frequency fixed from the outset and registers a frequency band received by a light receiver with a single modulating frequency fixed in pulse light emitted from a light projector. However, if there is any noise (disturbance light) which can be detected by the light receiver, there is the possibility of its irregular amplitude causing it to pass through a filter or tuning circuit subsequent to the light receiver to produce a light reception output (indicating that there is no intruder), even if such noise has no frequency which is the same as or similar to the modulating frequency for the infrared rays. Thus, to solve this problem, an infrared type warning device using double-modulated infrared pulse light as disclosed in Japanese Laid-Open Patent Publication No. 21132/1984 uses an emitter (light projector) of the type which modulates the amplitude of a high frequency signal by means of a low frequency signal, while employing in a receiving circuit a phase locked loop (hereinafter referred to as PLL) which follows the frequency and phase of an input signal, with the intention of providing a highly sensitive receiving device. That is, on the light receiving side of this apparatus, pulse light emitted from the light projector is highly sensitively received by a tuned amplifier and PLL, but it has been impossible to switch modulating and demodulating frequencies for high and low frequencies set in a warning device to other values.